1. Field of the Invention
This invention relates to an image forming apparatus, such as a copier, a printer, or a facsimile machine, having an endless belt for carrying an image or for conveying an image forming medium, and forming an image using an electrophotographic method. This invention also relates to a rotary body detection device.
2. Description of Related Art
Conventionally, among image forming apparatuses using an electrophotographic method or an electrostatic recording method in an image forming process, a type using a cylindrical photosensitive drum as an image carrier is commonly known, wherein the photosensitive drum is surrounded with surrounding devices (e.g, a charging unit, an exposing means, a developing unit, a cleaning unit) and a transfer material conveying mechanism (transfer material conveying means) or the like.
Presently, in means to improve the functions for the photosensitive drum and the transfer material conveying mechanism or the like, instead of using a photosensitive drum, an image forming apparatus using an endless belt for a photosensitive body serving as an image carrier, or an image forming apparatus using an endless belt for a transfer material conveying mechanism, has been developed.
Although numerous functions have been improved with the image forming apparatus using the endless belt for a photosensitive body or a transfer material conveying mechanism, such image forming apparatus requires a speed control means for controlling change in moving speed of the endless belt caused by internal warming of the image forming apparatus body or change in the environmental temperature, which are distinctive problems for such belt mechanism.
Typically, the tolerance for an outer diameter of a driving roller for driving the endless belt is strictly defined so that the moving speed of the endless belt precisely conforms with a recording position. However, due to the internal warming of the image forming apparatus body or change in the environmental temperature, the driving roller expands or contracts to change the moving speed of the endless belt, and results in problems such as deterioration in the precision of recording position and degrading of image quality.
Therefore, conventionally, methods such as using a material resistant to heat expansion for the driving roller, or measuring the temperature surrounding the driving roller and anticipating the diameter of the roller, have been used as a speed controlling means for controlling the moving speed of the endless belt.
However, the aforementioned methods are unable to satisfactorily prevent deviation in color or unevenness in density caused by a slight deviance in position.
In means to control the change in speed caused by eccentricity of the driving roller, Japanese Patent Publication (Kokai) No. Hei 4-172376, Hei 4-234064, or Hei 4-234064 show an example where a rotary encoder is disposed on an axis of a driven roller being rotatively driven by an endless belt to detect the rotation angle speed and to control the rotary speed of the motor of the driving roller on the basis of the detected result.
The method of detecting the moving speed of the endless belt from the driven roller is effective not only for controlling the change in speed from the eccentricity of the driving roller, but is also effective for controlling the change in speed from thermal expansion.
Nevertheless, using a precise rotary encoder leads to a problem of considerable cost increase.
Accordingly, this invention has as an object to detect change in moving speed of the endless belt from thermal expansion more simply and precisely.
For accomplishing the foregoing object, a representative structure of this invention is an image forming apparatus comprising: an endless belt for carrying an image or for conveying an image forming medium; a driving roller connecting across the endless belt for driving the endless belt; a driven roller connecting across the endless belt for being driven in correspondence to movement of the endless belt; an image forming means for forming an image to the endless belt or to a medium conveyed by the endless belt; a speed detection means for detecting moving speed of the endless belt; and a speed control means for controlling the moving speed of the endless belt based on a detection result from the speed detection means; wherein the speed detection means generates one pulse per rotation of the driven roller being driven in correspondence to the endless belt.
Thus structured, a mechanical precision of a pulse signal generating portion (e.g, the eccentricity of the rollers) can be ignored to allow a simpler and more precise detection in the movement speed of the endless belt.
The speed detection means can easily emit a signal by generating a signal from a notch or a perforation formed at a portion of the driven roller.
Position of the speed detection means and an axis of the driven roller can be secured and a signal can constantly be generated stably so that the rotation of the driven roller can be steadily detected by forming the notch or the perforation on an axis end portion of the driven roller, fixing the speed detection means to an axial bearing of the driven roller or to an axial bearing securing member for securing the axial bearing, and generating a signal by passage and blockage of light from the notch or the perforation formed on the axis end portion of the driven roller.
The speed control means being based on a pulse count of the driven roller when the value of movement of the endless belt, the driven roller, and the driving roller is substantially equal to a common multiple of a peripheral length of the driven roller and a peripheral length of the driving roller can prevent error from change of speed caused by the eccentricity of the driven roller and enhance precision in controlling the endless belt.
The speed detection means being based on a pulse count of the driven roller when the value of movement of the endless belt, the driven roller, and the driving roller is substantially equal to a common multiple of a peripheral length of the driven roller and a peripheral length of the endless belt can prevent error from change of speed caused by uneven thickness or the like of the endless belt, and enhance precision in controlling the endless belt.
Further, the driven roller having a coefficient of linear expansion substantially equal to a coefficient of linear expansion for a member which defines the interval of the image forming means can prevent deviation in color when actually forming an image since a movement value of the driven roller when the belt speed is detected as moving slower than the actual speed thereof can be substantially balanced with a value of the thermal expansion from the position of the image forming means, and when a plurality of image forming means are disposed with a predetermined interval on the endless belt.
In a rotary body detection device serving to detect the speed of a rotary body and comprising a rotary body and a sensor for generating a signal by passage and blockage of light, a rotation speed of the rotary body can be easily detected by forming a notch or a perforation at a portion of the rotary body, and by allowing the sensor to generate a signal by passage and blockage of light from the notch or the perforation.
The sensor can be precisely positioned to allow precise generation of a signal by defining a relative position between an axial bearing of the rotary body or the axial bearing securing member for securing the axial bearing.